A conventional printer, such as an ink jet printer, forms characters or graphic images by printing closely spaced pixels which overlap. The pixels are printed on a record medium as a print head carrying a plurality of ink jet nozzles is moved across the record medium in a line scan direction. Between line scans, a stepper motor moves the record medium in a direction transverse to the line scan direction. The number and position of the nozzles may vary but, generally speaking, the nozzles are capable of printing vertically aligned and overlapping pixels with a given center-to-center spacing or resolution. Print heads, such as that shown in U.S. Pat. No. 4,972,270 are capable of printing pixels on 1/300 inch centers. All points addressable printing using this head would be possible if one employed a stepper motor capable of moving the record medium in increments of 1/300 inch. Because of design problems and the cost of such motors it would be preferable to be able to use a stepper motor that moved the paper in larger increments.
Most printers are designed to provide either a one-sixth or one-eighth inch line-to-line spacing since these spacings were almost uniformly used in mechanical and electro-mechanical typewriters. To obtain exactly one-sixth or one-eighth inch line-to-line spacing using a stepper motor, the motor must be specially designed so that it advances the record medium 1/n inch in response to each stepping pulse where n is some multiple of the product of 6 and 8. For example, a stepper motor might be designed to advance the record medium one inch in response to 96 pulses. Such a motor would move the record medium 1/8 inch in response to 12 pulses or 1/6 inch in response to 16 pulses. When printing pixels on 1/300 inch centers, this arrangement does not allow advancing the record medium an arbitrary number of pixels. One approach is to select a step motor and gear train that increments the record medium 1/150 inch in response to each stepping pulse. In this case however, the 1/6 inch line increments require the motor to advance 25 steps for each line. Four phase step motors are less accurate when stepped an odd number of steps because of mechanical/magnetic tolerances. If an even number of steps are taken to advance the paper, these tolerances tend to cancel. A further improvement in accuracy is obtained if the number of steps taken to advance the paper is evenly divisible by 4. This is because the move will always start and finish on the same winding (or phase). Each different winding also has different magnetic tolerances which contribute to move inaccuracy. If the move starts and finishes on the same winding this inaccuracy is reduced.
The present invention permits use of a commercially available motor and a stepping increment greater than the desired pixel-to-pixel spacing to obtain all points addressable printing while at the same time permitting exact or nearly exact line-to-line spacing of 1/6 inch, where the move's total steps are evenly divisible by 4, or 1/8 inch, where the move's total number of steps are divisible by 4 90% of the time and an even number of steps 10% of the time, yielding improved accuracy.